Just to add to the others good suggestions. One possible reason is that one end of the connection has an access-list applied to the interface where the interesting traffic comes from.
Hence if one side initiates the connection and traffic flows from one side to other it works fine because the return traffic is not subject to the access-list as it is stateful traffic. However traffic may be stopped from being initiated on the other side because of the access-list.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...